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Taxonomy for the western fence lizard has been under much debate. S. occidentalis belongs in the order Squamata (snakes and lizards) and the suborder Iguania. The family in which it belongs is still under scrutiny. The family Phrynosomatidae, along with seven other families, used to be included in the single family Iguanidae, until Frost and Etheridge's (1989) analysis of iguanian systematics suggested the family be divided.[2] Some literature, however, still places the phrynosomatids in Iguanidae.

Western fence lizards measure 5.7-8.9 cm (snout-vent length)[3] and a total length of about 21 cm.[4] They are brown to black in color (the brown may be sandy or greenish) and have black stripes on their backs, but their most distinguishing characteristic is their bright blue bellies. The ventral sides of the limbs are yellow.[5] These lizards also have blue patches on their throats. This bright coloration is faint or absent in both females and juveniles. In some populations the males also display iridescent, bright turquoise blue spots on the dorsal surface. The scales of S. occidentalis are sharply keeled, and between the interparietal and rear of thighs, there are 35-57 scales.[3]

Many other lizards have similar bright-blue coloring. The eastern fence lizard, S. undulatus, instead of having one large patch on its throat, has two small patches.[3] The sagebrush lizard, S. graciosus, lacks yellow limbs and has smaller dorsal scales.[3]S. occidentalis also resembles the side-blotched lizard, Uta stansburiana. However, the axilla of U. stansburiana usually has a black spot behind it and it has a complete gular fold.[3]

Although California is the heart of the range of this lizard, it is also found in eastern and southwestern Oregon (some populations are found even north of Seattle, WA), as well as in the Columbia River Gorge, southwestern Idaho, Nevada, western Utah, northwestern Baja California, Arizona, and some of the islands off the coast of both California and Baja California. There is also an isolated population in the Northwestern Tualitin Valley, around Henry Hagg Lake.[6]

The western fence lizard occupies a variety of habitats. It is found in grassland, broken chaparral, sagebrush, woodland, coniferous forest, and farmland, and occupies elevations from sea level to 10,800 ft.[3] They generally avoid the harsh desert and are often found near water.

As of now, the western fence lizard is listed as unprotected, and no conservation restrictions apply.[5]

These lizards are diurnal, and are commonly seen sunning on paths, rocks, and fence posts, and other high places, which makes them an easy target for predation by birds and even some mammals, such as shrews. They protect themselves by employing their fast reflexes, which are common in many other lizards including biting and possibly defecating on the predator.[5] They can change color from light grey or tan to nearly jet black, but they probably use this ability for the purpose of thermoregulation while basking and not as a means to camouflage themselves.

The western fence lizard eats spiders and insects such as beetles, mosquitoes, and various types of grasshoppers.

Like most other lizards, S. occidentalis goes through a period of hibernation during the winter. The length of time and when they emerge varies depending on climate. During the mating season, adult males will defend a home range.[4]

Western fence lizards mate in the spring, and do not breed until the spring of their second year. Females lay one to three clutches of three to 17 eggs (usually eight) between April and July. The eggs hatch in August.[3][4]

Studies have shown Lyme disease is lower in areas where the lizards occur. When ticks carrying Lyme disease feed on these lizards' blood (which they commonly do, especially around their ears), a protein in the lizard's blood kills the bacterium in the tick that causes Lyme disease. The blood inside the ticks' gut is therefore cleansed and no longer carries Lyme disease.[7]